This invention relates to a method and handheld apparatus for the measurement of liquid density in a static or agitated column of liquid. This invention has particular application to the measurement of the density or specific gravity of sulfuric acid in a flooded lead-acid battery as is commonly used in the motive power, telecommunications, uninterruptible power supply, nuclear and automotive industries.
As can be seen by references in this field of invention, there are many uses for the measurement of liquid density. There are also many means by which liquid density can be measured. The specific field of this invention pertains to the measurement of liquid density by reading the differential pressure at two vertically disposed positions. It is an accepted principle that the differential pressure between two points vertically disposed in a column of liquid is the product of the vertical distance between them and the density of the liquid. If the distance is kept constant, the differential pressure will be a linear function of density.
While the use of this invention applies itself to many different applications, it is specifically useful in the measurement of sulfuric acid density in a flooded lead-acid battery. Regular maintenance on this type of battery requires the frequent measurement of voltage, temperature and liquid specific gravity. In many cases, as many as 240 individual batteries comprise a system in one location. An added complication to the requirements of the present invention includes the limited space between the top of one battery and the bottom of another in a battery rack. Another is the requirement to take the measurement in a draw tube located at the comer of most batteries, having a diameter as small as 1/4 inch and extending up to ten inches vertically into the battery's liquid.
In review of prior art, only three different means of liquid density measurement have been incorporated into a handheld device. These three are: 1) The float style hydrometer; 2) Frequency measurement of a vibrating liquid sample; and 3) Optical refraction. The following references are cited as examples of these means of measurement: U.S. Pat. Nos. 3,451,273, 3,754,445 and 4,132,110.
The disadvantages of these devices are many. The float style hydrometer and the vibration methods require drawing samples into a separate chamber for the reading to take place. This is both time consuming and unsafe when dealing with hazardous chemicals like sulfuric acid. The optical refraction method has been adapted so as not to require the removal of a sample but is not designed such that it can enter the draw tube of a battery. The float style hydrometer has been adapted for use in this application but requires a reading from a tightly graduated scale which is prone to calibration inaccuracies as well as human reading error. The mechanical nature of this device makes it unadaptable to electronic manipulation such as data logging for saving or communicating information to a central processing unit. The vibration device is also prone to error due to its tendency to be fouled by gas bubbles in the tubes.
Other references cited give specific attention to a device known as a "gas bubbler". The original "gas bubbler", cited by references U.S. Pat. Nos. 1,457,406 and 1,460,134, do not provide for a handheld device nor would either of them be well applied to a handheld device because of their analog readings and inability to provide the degree of accuracy required by many applications. All other developments in the "gas bubbler" have not included the necessary elements to allow portability and accuracy in the same invention. References U.S. Pat. Nos. 2,394,549, 2,577,548 and 4,393,705 are further examples of these inadequacies.
Unlike other references cited, the present invention provides numerous advantages over previous methods. None of the references cited overcome the disadvantage of pulling a sample of liquid into a separate chamber except for one device which requires a much larger diameter for access to the chamber of the liquid being measured. Alternatively, a particular embodiment of the apparatus can access liquid chambers with openings down to 1/4 inch in diameter. This is particularly required for accessing sulfuric acid in a battery or for being directly interchangeable with a typical float style hydrometer. Surpassing the typical float style hydrometer, the present invention can access and measure density in a column of liquid down to 1/4 inch in diameter without drawing liquid and can do so in a physical environment where there is limited clearance with another battery or other object above.
Further, the ability to take almost instantaneous readings without extracting a liquid sample has two advantages. First, a significant time savings can be seen in a job requiring the reading of numerous liquid samples. Second, the handling of hazardous chemicals can be avoided. This is particularly important in the example of sulfuric acid in a flooded lead-acid battery.
Another particular embodiment of the present invention provides for a higher degree of accuracy in a handheld device. It is understood by those skilled in the art that the characteristics of how the gas bubbles form at the opening orifice of a bubble tube in a liquid can significantly affect the accuracy of the differential pressure being measured by the differential pressure transducer. As the orifices move closer together for compactness, the percentage effects of bubble formation to overall accuracy become greater. Further, the constant removal, drying and rewetting of the orifices are cause for more error as would be characteristic of a handheld device.
Agitation, viscosity and surface tension have all been cited in previous inventions as being reasons for inaccuracies as these all affect how consistently bubbles are formed and released at the opening. References U.S. Pat. Nos. 2,668,438 and 2,755,669 both address this problem by offering embodiments which are intended to dampen pressure fluctuations and achieve greater consistency in bubble formation. The embodiments described in these references do not address the requirement to produce accurate results in a bubble tube arrangement that is small enough to enter a 1/4 inch diameter draw tube as would be found on a flooded lead-acid battery. Further, these references offer embodiments that are to sensitive to slight changes in angle of the bubble tube opening orifice as would be characteristic of a handheld device.
Another reference, U.S. Pat. No. 4,949,572, addresses the bubble formation problem by taking a reading only after stopping gas flow at a controlled point. This would not be practical in a handheld device due to the moveability of the probe. A preferred embodiment of this invention achieves the required accuracy with fewer components resulting in higher reliability and lower cost.
Further, most density measurement applications do not require a very high degree of accuracy. The present invention can sustain readings of liquid density with an accuracy and repeatability of up to plus or minus 0.002 grams per cc. This meets or exceeds the requirements of most density measurement applications.